The invention relates to the stabilization of a laser, specifically a semiconductor diode laser of the type commonly used in opto-electronics, mostly as so-called pump lasers for fiber amplifiers in the field of optical communication, e.g. for Erbium-doped fiber amplifiers. Such lasers are designed to provide a narrow-bandwidth optical radiation with a stable power output in a given frequency band. In particular, the invention concerns an improved design of the external cavity exhibiting a significantly improved stability compared to prior art designs.
Semiconductor lasers of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the lasers are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are 980 nm lasers with a power output of 150 mW or more, which wavelength matches the 980 nm Erbium absorption line and thus achieves a low-noise amplification. InGaAs lasers have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to InGaAs lasers.
There are other examples of lasers of other wavelengths and materials for which the present invention is applicable. Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (or FP) lasers. Three main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP and multiquantum-well InGaAs lasers at 1480 nm; strained quantum-well InGaAs lasers at 980 nm; and GaAlAs lasers at 820 nm.
One of the problems occurring when using semiconductor lasers for the above purpose is their wavelength and power output instability which, though small, still affects the amplification sufficiently to look for a solution to the problem. This problem is already addressed in Erdogan et al. U.S. Pat. No. 5,563,732, entitled xe2x80x9cLaser Pumping of Erbium Amplifierxe2x80x9d, which describes the stabilization of a pump laser of the type described above by use of a Bragg grating in front of the laser. This grating forms an external cavity with the laser. The laser bandwidth is broadened and stabilized by the reflection from the grating. It is believed that the laser operation in so-called xe2x80x9ccoherence-collapsexe2x80x9d is obtained by providing sufficient external optical feedback, here from a fiber Bragg grating within the optical fiber into which the laser light is usually coupled. This grating is formed inside the guided-mode region of the optical fiber at a certain distance from the laser. Such a fiber Bragg grating is a periodic structure of refractive index variations in or near the guided-mode portion of the optical fiber, which variations are reflecting light of a certain wavelength propagating along the fiber. The grating""s peak-reflectivities and reflection bandwidths determine the amount of light reflected back into the laser.
Ventrudo et al. U.S. Pat. No. 5,715,263, entitled xe2x80x9cFibre-grating-stabilized Diode Laserxe2x80x9d describes an essentially similar approach for providing a stabilized laser, showing a design by which the laser light is coupled to the fiber by focussing it through a fiber lens. Again, a fiber Bragg grating is provided in the fiber""s guided mode portion, reflecting part of the incoming light back through the lens to the laser. To summarize, when positioning a fiber Bragg grating beyond the coherence length of the laser and when the laser gain peak is not too far from the Bragg grating""s center wavelength, it is understood that the laser in coherence collapse operation is forced to operate within the optical bandwidth of the grating and thus is wavelength-stabilized. Additionally, low-frequency power fluctuations seem to decrease by the effect of induced high-frequency multi-mode operation.
In general, the above-described prior art devices must have a length of the external cavity, i.e. the optical fiber, somewhere at least between 0.5 and 1 m, to definitely assure coherence collapse laser operation. For some even up to 2 m long optical fibers are required. This rather long fiber determines the size of the laser source and makes it comparatively bulky.
Some types of semiconductor lasers, especially others than those in the above mentioned patents, e.g. lasers having a narrow spectral gain width, are seen to exhibit instability at certain operating conditions, in particular undesirable switching from multi-mode to single-mode operation within the grating bandwidth. This mode switching (coherence collapse occurs in both cases) results in a fluctuation of the effective laser output which in turn produces noise, thereby negatively affecting or actually disturbing the amplification process. The mode-switching problem is aggravated by new generations of semiconductor laser diodes having at least twice as much output power than the lasers in the Ventrudo or Erdogan patent and the desire of the industry to have wavelength stabilization for all possible operating conditions of a laser.
Other techniques have been proposed to correct fiber amplifier output power fluctuations, e.g. active methods to control the variations in the fiber amplifier output by feedback of an electric signal effecting a correction of the laser power. A further solution is an electronic dithering circuitry forcing the laser to operate multimode, described by Heidemann et al. in U.S. Pat. No. 5,297,154, entitled xe2x80x9cFiber-Optic Amplifier with Feedback-Insensitive Pump Laserxe2x80x9d. However, the need for active components for these solutions add complexity and cost.
For a quite different purpose, Fischer et al. describe in xe2x80x9cHigh-dimensional Chaotic Dynamics of an External Cavity Semiconductor Laserxe2x80x9d, Phys. Review Letters, Vol. 73, No. 16, October 1994, pp. 2188-2191, an experimental laser setup with an external T-shaped cavity comprising a beam splatter and high reflecting gold mirrors at each of the two ends of the cavity""s two arms. Though this layout shows an external two-cavity arrangement, it is absolutely unsuitable for the purpose of the present invention, since the lengths chosen for the arms of the cavity and the reflectivities of the laser""s exit facet and the above-mentioned gold mirrors are selected to avoid the coherence collapse just the opposite of the present invention, where coherence collapse is a prerequisite.
Also in a very different field, Wang Xianghyang et al. disclose a xe2x80x9cTheoretical and Experimental Study on the Fabrication of Double Fiber Bragg Gratingsxe2x80x9d in the journal Optical Fiber Technology: Materials, Devices and Systems, Vol. 3, No. 2, pp. 189-193. Double gratings are provided at the same location within the fiber and this xe2x80x9cchirpedxe2x80x9d grating is said to widen the transmission spectrum of the fiber. Again, this publication does nowhere address the problem that the invention intends to solve.
Thus, it is the main object of the invention to devise a simple and reliable laser source layout, especially for pump lasers in optical fiber communication systems, that provides a stable output under all operating conditions. A specific object is to avoid the detrimental mode switching of the laser, even for a laser output power of more than 150 mW, and thus increase the stability of the output of high power laser sources. Output stability shall be achieved for high optical power having reduced low frequency noise, wavelength stability and high side lobe suppression outside the fiber Bragg grating bandwidths.
A further object is to allow maximum flexibility for choosing the lasers parameters without running into stability problems.
A still further object is to avoid any further complexity and keep the number of additional components of the laser source within a laser pumped optical amplifier to a minimum.
A particular object is to create a stabilized laser source of reduced size by using a significantly shortened external cavity region.
In brief, to solve the problems addressed above, the present invention does not use a single grating or cavity in front of the laser, but a plurality of appropriately arranged cavities. These cavities are preferably arranged in series, but can also be arranged in parallel. If the lengths of the cavities, their reflectivities, and their peak wavelengths are chosen accordingly, the laser is forced to operate multimode under all or practically all operating conditions.
Whereas one single grating is known to act as a wavelength broadening and stabilization element, it is understood that multiple cavities according to the invention impose a useful destabilization only within the compound bandwidth of all gratings high enough to force the laser into multimode operation. In a way, this phenomenon may be named as a xe2x80x9cphotonic ditherxe2x80x9d with a similar effect as an electronic dither, but by means of passive components only.
Another advantage of the invention is that the total length of the external cavities can be reduced to less than the length of the prior art designs. This provides for smaller laser sources.
One preferred embodiment according to the invention has a first reflector in front of a semiconductor laser diode forming a first cavity and, at an optimized distance, a second reflector in front of the first one, forming a second cavity between the first and the second reflector within the optical fiber. The peak wavelength of the second reflector may be chosen close, but not necessarily identical to that of the first reflector. Also, a certain offset of the peak wavelengths and/or bandwidths of the two reflectors may improve performance. Further, any one or both cavities may be shorter than the coherence length of the laser diode.
Preferably, the phase relationship between the two reflectors is chosen such that the resulting waves or fields arexe2x80x94statistically seenxe2x80x94practically out of phase.
The reflectors can be of any kind suitable for the desired purpose; they are preferably provided as Bragg gratings within the optical fiber, which simplifies their making and avoids the need to have any parts or components added.
A preferred method for providing the desired plurality of cavities is to establish them by simultaneously producing the desired Bragg gratings within the optical fiber. This keeps the additional efforts to fabricate the additional cavities at a minimum and, at the same time, provides for close tolerances of the desired layout.